With its neo-Georgian mansion, acres of gardens in Regent’s Park and access to high society, the job of US Ambassador to London has many attractions.

But candidates for arguably the most glamorous posting on the map should know that it is not a position to be secured by diplomatic skills alone.

Like every president before him, Barack Obama will fill about 30 per cent of diplomatic posts with friends and donors — and the bigger the donation, the better the posting. Until now, the question of how much is enough to land the job in London, Paris or, say, a Caribbean